The present invention relates to ultrasonic transducer arrays. It is specifically directed to an electronic method of translating the focal region produced by a zone-plate array.
The past several years have witnessed a dramatic increase in the use of ultrasonic imaging in medical applications. It has become possible to view internal organs through the use of high-frequency acoustic waves that are introduced into the subject's body. The received echoes can give trained personnel a large amount of information without the necessity of employing X-rays. The use of ultrasonic imaging does not appear to be accompanied by the dangers that the use of X-rays has, and in some cases the information derived from acoustic imaging exceeds that obtained with X-rays.
Much of the work in acoustic imaging has been directed to focusing the acoustic energy. Focusing acoustic waves is similar in some respects to focusing light waves; acoustic lenses actually exist and have been used. However, acoustic waves appear also to lend themselves to focusing by means of constructive interference from discrete sources of acoustic energy. Acoustic energy is focused on a region if waves from a number of discrete sources constructively interfere in that region. Accordingly, if the relative phases of several sources can be adjusted so that they all arrive with the same phase at the desired point, acoustic focusing at that point has been achieved. This type of focusing naturally brings to mind the phased-array organizations employed in some sophisticated radar systems, and the phased-array organization has indeed found its way into the ultrasonic field. For instance, a linear array of transducers whose relative phases are controlled by tapped delay lines is described in an article by Maginness et al "State of the Art in Two-Dimensional Ultrasonic Transducer Array Technology," Medical Physics, September-October 1976, pp. 312-318. This type of arrangement is desirable in that it allows totally electronic focusing of the ultrasonic system. The same article also illustrates two-dimensional arrays, and the design and construction of such arrays is exemplified by the illustrated embodiment in U.S. Pat. No. 3,979,711 to Maginness et al.
Such arrays are quite desirable because the focusing is performed totally electronically, requiring no moving parts and allowing a scan rate that is rapid compared to the rates possible with mechanical focusing. However, they also tend to be somewhat expensive because a variable delay line must be provided for each of the elements of the array, and such delay lines are somewhat expensive particularly in quantity. Furthermore, a fair amount of information must be managed in such a system because the amount of delay required of each delay line must be stored. Furthermore, the typical phased-array arrangement often results in images that can be difficult to interpret because the resolution of the system varies with the distance to the focal region.
An arrangement that avoids some of these problems as described by Farnow and Auld in "An Acoustic Phase Plate Imaging Device," Acoustical Holography, Vol. 6, pp. 259-73 (N. Booth, ed., Plenum Press, 1975). This article describes the use of a Rayleigh-Wood phase-reversal zone plate in which concentric annular transducer elements are spaced so that the acoustic waves all constructively interfere at the desired focal point in alternate elements are excited with opposite phases. Focusing in the depth direction is then accomplished by varying the frequency, which varies the distance to the point at which the ultrasonic waves from all the sources are in phase. This type of arrangement has two significant advantages. One is its tremendous simplicity as compared with the normal phased array. The second advantage is that the resolution achieved with this technique is substantially independent of the distance to the focal region. It can be appreciated that this is a great advantage for interpreting the images produced.
Accompanying the simplicity of the two-dimensional zone-plate arrangement, however, is the necessity for mechanically moving a two-dimension zone-plate transducer array if transverse motion of the focal region is desired. This is a significant drawback in comparison with the phased-array arrangement, particularly where real-time imaging is desired.
It is accordingly the object of the present invention to provide the simplicity, low cost, and uniform resolution of a zone-plate system while permitting transverse motion of the focal region to be accomplished electronically.